Cascade Overflow Valve Instead of using an electric
supply pump, this fuel system uses a gear supply
pump located inside the rear of the high pressure
pump. The pump is driven by an eccentric on the end
of the high pressure pump shaft. The gear pump
draws fuel from the fuel tank through the fuel filter.
The pressurized outlet side of the gear pump provides
pressurized fuel to a branched circuit internal
to the high pressure pump flange, which supples
both the fuel quantity solenoid and the cascade overflow
valve. Because the gear pump increases fuel
flow and pressure as the engine rpm increases, the
pressure is regulated by the cascade overflow valve.
The cascade overflow valve and gear supply pump
are not serviced independently of the high pressure
pump.
The cascade overflow valve has two functions:
² Regulation of lubrication fuel to the internal
moving parts of the high pressure pump
² Regulation of the fuel pressure being supplied to
the fuel quantity solenoid
The cascade valve has a machined center piece
that has three drillings. One for overflow, one for
lubrication and one for supply. The valve works in
three stages based on the pressure entering the inlet
of the valve.
Stage 1 When the fuel pressure entering the tip of
the cascade valve is between 0 and 3 bar (44 psi), the
spring force is not overcome and fuel only flows
through the center drilling. This drilling always
allows fuel flow through to the pump center ring and
lubricates the pump bushings and internal moving
parts. This circuit also allows air to bleed during initial
cranking and returns the air to the fuel tank.
The cascade valve is only in stage one during cranking.
Stage 2 When the fuel entering the cascade valve
exceeds 3 bar (44 psi), but is less than 5 bar (73 psi),
the center piece of the valve moves against the
spring force aligning another passage for lubrication
purposes. Stage 2 can be reached during cranking
and initial start up.
Stage 3 When fuel pressure exceeds 5 bar (73 psi),
the center of the valve aligns with the overflow passage.
This stage relieves the pressure into an overflow
circuit that sends the fuel back to the inlet side
of the gear pump which limits maximum fuel pressure
to 5 bar (73 psi). Lubrication fuel also continues
to flow though the other ports during this stage.
Excess is sent back to the fuel tank through the
return circuit.
High Pressure Pumping Plungers The fuel quantity
solenoid supples three high pressure pumping
chambers. The pumping chambers have one way
inlet valves that allow fuel to flow into the chambers.
The valves then close during compression of the fuel
and cause the high pressure fuel to overcome a ball
and angled seat outlet valve.
All three pumping chambers are tied together in
one circuit internal to the pump and provide high
pressure fuel up to 1600 bar (23,000 psi) through a
steel line, to the fuel rail.
The pump is driven at 1 :1 engine speed and is not
responsible for injection timing. The pump is only
responsible for providing high pressure fuel while the
ECM controls the injection timing.